Department Of Computer Science

ASSAM UNIVERSITY, SILCHAR

Submitted By Submitted To: Mrinal Kanti Paul Mr. B.S. Mena

6th Semester

Roll No.: 03

Transmission Media: Sender Receiver

Transmission medium

Cable or air

A transmission medium can be broadly defined as anything that can carry information

from a source to a destination. For example, the transmission medium for two people having

a dinner conversation is the air. The air can also be used to convey the message in a smoke

signal or semaphore. For a written message, the transmission medium might be a mail carrier,

a truck, or an airplane.

In data communications the definition of the information and the transmission

medium is more specific. The transmission medium is usually free space, metallic cable, or

fiber-optic cable. The information is usually a signal that is the result of a conversion of data

from another form.

In telecommunications, transmission media can be divided into two broad categories:

guided and unguided. Guided media include twisted-pair cable, coaxial cable, and fiber-optic

cable. Unguided medium is free space.

Figure: Classes of transmission media

Physical Layer Physical Layer

Transmission Media

Unguided (Wireless) Guided (Wired)

Infrared Radio

wave Fiber – optic

cable

Microwave Twisted – pair

cable Coaxial

cable

GUIDED MEDIA: Guided media, which are those that provide a conduit from one device to another,

include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of

these media is directed and contained by the physical limits of the medium. Twisted-pair and

coaxial cable use metallic (copper) conductors that accept and transport signals in the form

of electric current. Optical fiber is a cable that accepts and transports signals in the form of

light.

Twisted-Pair Cable: A twisted pair consists of two conductors (normally copper), each with its own

plastic insulation, twisted together, as shown in Figure.

One of the wires is used to carry signals to the receiver, and the other is used only as a

ground reference. The receiver uses the difference between the two.

In addition to the signal sent by the sender on one of the wires, interference (noise)

and crosstalk may affect both wires and create unwanted signals.

If the two wires are parallel, the effect of these unwanted signals is not the same in

both wires because they are at different locations relative to the noise or crosstalk sources

(e.g., one is closer and the other is farther). This results in a difference at the receiver. By

twisting the pairs, a balance is maintained.

Unshielded Versus Shielded Twisted-Pair Cable:

The most common twisted-pair cable used in communications is referred to as

unshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair cable for its

use called shielded twisted-pair (STP). STP cable has a metal foil or braided – mesh covering

that encases each pair of insulated conductors. Although metal casing improves the quality

of cable by preventing the penetration of noise or crosstalk, it is bulkier and more expensive.

Figure shows the difference between UTP and STP.

Categories

The Electronic Industries Association (EIA) has developed standards to classify

unshielded twisted-pair cable into seven categories. Categories are determined by cable

quality, with 1 as the lowest and 7 as the highest. Each EIA category is suitable for specific

uses.

Connectors:

The most common UTP connector is RJ45 (RJ stands for registered jack), as shown in

Figure. The RJ45 is a keyed connector, meaning the connector can be inserted in only one

way.

Performance:

One way to measure the performance of twisted-pair cable is to compare attenuation

versus frequency and distance. A twisted-pair cable can pass a wide range of frequencies.

However, Figure shows that with increasing frequency, the attenuation, measured in decibels

per kilometer (dB/km), sharply increases with frequencies above 100 kHz. Note that gauge is

a measure of the thickness of the wire.

Applications:

Twisted-pair cables are used in telephone lines to provide voice and data channels.

The local loop-the line that connects subscribers to the central telephone office---commonly

consists of unshielded twisted-pair cables.

The DSL lines that are used by the telephone companies to provide high-data-rate

connections also use the high-bandwidth capability of unshielded twisted-pair cables.

Local-area networks, such as lOBase-T and lOOBase-T, also use twisted-pair cables.

Coaxial Cable: Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted – pair

cable, in part because the two media are constructed quite differently. Instead of having two wires,

coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating

sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the

two. The outer metallic wrapping serves both as a shield against noise and as the second conductor,

which completes the circuit.

This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected

by a plastic cover.

Coaxial Cable Standards:

Coaxial cables are categorized by their radio government (RG) ratings. Each RG

number denotes a unique set of physical specifications, including the wire gauge of the inner

conductor, the thickness and type of the inner insulator, the construction of the shield, and

the size and type of the outer casing. Each cable defined by an RG rating is adapted for a

specialized function.

Coaxial Cable Connectors:

To connect coaxial cable to devices, we need coaxial connectors. The most common

type of connector used today is the Bayone-Neill-Concelman (BNe), connector. Figure shows

three popular types of these connectors: the BNC connector, the BNC T connector, and the

BNC terminator.

Performance:

As we did with twisted-pair cables, we can measure the performance of a coaxial

cable. We notice in Figure that the attenuation is much higher in coaxial cables than in

twisted-pair cable. In other words, although coaxial cable has a much higher bandwidth, the

signal weakens rapidly and requires the frequent use of repeaters.

Applications:

Coaxial cable was widely used in analog telephone networks where a single coaxial

network could carry 10,000 voice signals. Later it was used in digital telephone networks

where a single coaxial cable could carry digital data up to 600 Mbps. However, coaxial cable

in telephone networks has largely been replaced today with fiber-optic cable.

Cable TV networks (see Chapter 9) also use coaxial cables. In the traditional cable TV

network, the entire network used coaxial cable. Later, however, cable TV providers replaced

most of the media with fiber-optic cable; hybrid networks use coaxial cable only at the

network boundaries, near the consumer premises. Cable TV uses RG-59 coaxial cable.

Fiber-Optic Cable: A fiber-optic cable is made of glass or plastic and transmits signals in the form of light.

Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by

a cladding of less dense glass or plastic. The difference in density of the two materials must be such

that a beam of light moving through the core is reflected off the cladding instead of being refracted

into it.

Propagation Modes:

Current technology supports two modes (multimode and single mode) for propagating

light along optical channels, each requiring fiber with different physical characteristics.

Multimode can be implemented in two forms: step-index or graded-index.

Multimode: Multimode is so named because multiple beams from a light source

move through the core in different paths. How these beams move within the cable depends

on the structure ofthe core, as shown in Figure.

In multimode step-index fiber, the density of the core remains constant from the

center to the edges. A beam of light moves through this constant density in a straight line

until it reaches the interface of the core and the cladding. At the interface, there is an abrupt

change due to a lower density; this alters the angle of the beam's motion. The term step index

refers to the suddenness of this change, which contributes to the distortion of the signal as it

passes through the fiber.

A second type of fiber, called multimode graded-index fiber, decreases this distortion

of the signal through the cable. The word index here refers to the index of refraction. As we

saw above, the index of refraction is related to density. A graded-index fiber, therefore, is one

with varying densities. Density is highest at the center of the core and decreases gradually to

its lowest at the edge.

Single-Mode: Single-mode uses step-index fiber and a highly focused source of light

that limits beams to a small range of angles, all close to the horizontal. The single – mode fiber

itself is manufactured with a much smaller diameter than that of multimode fiber, and with

substantially lower density (index of refraction). The decrease in density results in a critical

angle that is close enough to 90° to make the propagation of beams almost horizontal. In this

case, propagation of different beams is almost identical, and delays are negligible. All the

beams arrive at the destination "together" and can be recombined with little distortion to the

signal.

Fiber Sizes

Optical fibers are defined by the ratio of the diameter of their core to the diameter of

their cladding, both expressed in micrometres. Note that the last size listed is for single-mode

only. The common sizes are shown in Table.

Cable Composition:

The outer jacket is made of either PVC or Teflon. Inside the jacket are Kevlar strands

to strengthen the cable. Kevlar is a strong material used in the fabrication of bulletproof vests.

Below the Kevlar is another plastic coating to cushion the fiber. The fiber is at the center of

the cable, and it consists of cladding and core.

Fiber – Optic Cable Connectors:

There are three types of connectors for fiber-optic cables. The subscriber channel (SC)

connector is used for cable TV. It uses a push/pull locking system. The straight-tip (ST) connector

is used for connecting cable to networking devices. It uses a bayonet locking system and is

more reliable than SC. MT-RJ is a connector that is the same size as RJ45.

Performance:

The plot of attenuation versus wavelength in Figure shows a very interesting

phenomenon in fiber-optic cable. Attenuation is flatter than in the case of twisted-pair cable

and coaxial cable. The performance is such that we need fewer (actually 10 times less)

repeaters when we use fiber-optic cable.

Applications:

Fiber-optic cable is often found in backbone networks because its wide bandwidth is

cost-effective. Today, with wavelength-division multiplexing (WDM), we can transfer data at

a rate of 1600 Gbps.

Some cable TV companies use a combination of optical fiber and coaxial cable, thus

creating a hybrid network. Optical fiber provides the backbone structure while coaxial cable

provides the connection to the user premises.

Local-area networks such as 100Base-FX network (Fast Ethernet) and 1000Base-X also

use fiber-optic cable.

Advantages and Disadvantages of Optical Fiber:

Advantages Fiber-optic cable has several advantages over metallic cable (twisted –

pair or coaxial).

Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and

hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and

bandwidth utilization over fiber-optic cable are limited not by the medium but by the

signal generation and reception technology available.

Less signal attenuation. Fiber-optic transmission distance is significantly greater than

that of other guided media. A signal can run for 50 km without requiring regeneration.

We need repeaters every 5 km for coaxial or twisted-pair cable.

Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-

optic cables.

Resistance to corrosive materials. Glass is more resistant to corrosive materials than

copper.

Light weight. Fiber-optic cables are much lighter than copper cables.

Greater immunity to tapping. Fiber-optic cables are more immune to tapping than

copper cables. Copper cables create antenna effects that can easily be tapped.

Disadvantages There are some disadvantages in the use of optical fiber.

Installation and maintenance. Fiber-optic cable is a relatively new technology. Its

installation and maintenance require expertise that is not yet available everywhere.

Unidirectional light propagation. Propagation of light is unidirectional. If we need

bidirectional communication, two fibers are needed.

Cost. The cable and the interfaces are relatively more expensive than those of other

guided media.

UNGUIDED MEDIA: WIRELESS: Unguided media transport electromagnetic waves without using a physical conductor.

This type of communication is often referred to as wireless communication. Signals are

normally broadcast through free space and thus are available to anyone who has a device

capable of receiving them.

Figure shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900

THz, used for wireless communication.

Unguided signals can travel from the source to destination in several ways: ground

propagation, sky propagation, and line-of-sight propagation.

In ground propagation, radio waves travel through the lowest portion of the

atmosphere, hugging the earth. These low-frequency signals emanate in all directions from

the transmitting antenna and follow the curvature of the planet. Distance depends on the

amount of power in the signal: The greater the power, the greater the distance. In sky

propagation, higher-frequency radio waves radiate upward into the ionosphere where they

are reflected back to earth. This type of transmission allows for greater distances with lower

output power.

In line-or-sight propagation, very high-frequency signals are transmitted in straight

lines directly from antenna to antenna. Antennas must be directional, facing each other, and

either tall enough or close enough together not to be affected by the curvature of the earth.

Line-of-sight propagation is tricky because radio transmissions cannot be completely focused.

The section of the electromagnetic spectrum defined as radio waves and microwaves is

divided into eight ranges, called bands, each regulated by government authorities. These

bands are rated from very low frequency (VLF) to extremely high frequency (EHF).

We can divide wireless transmission into three broad groups: radio waves,

microwaves, and infrared waves.

Radio Waves: Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally

called radio waves; waves ranging in frequencies between 1 and 300 GHz are called

microwaves. However, the behaviour of the waves, rather than the frequencies, is a better

criterion for classification.

Radio waves, for the most part, are omnidirectional. When an antenna transmits radio

waves, they are propagated in all directions. This means that the sending and receiving

antennas do not have to be aligned. A sending antenna sends waves that can be received by

any receiving antenna. The omnidirectional property has a disadvantage, too. The radio waves

transmitted by one antenna are susceptible to interference by another antenna that may

send signals using the same frequency or band. Radio waves, particularly those waves that

propagate in the sky mode, can travel long distances. This makes radio waves a good

candidate for long-distance broadcasting such as AM radio.

Omnidirectional Antenna:

Radio waves use omnidirectional antennas that send out signals in all directions. Based

on the wavelength, strength, and the purpose of transmission, we can have several types of

antennas.

Applications:

The omnidirectional characteristics of radio waves make them useful for multicasting,

in which there is one sender but many receivers. AM and FM radio, television, maritime radio,

cordless phones, and paging are examples of multicasting.

Microwaves: Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves.

Microwaves are unidirectional. When an antenna transmits microwave waves, they can be

narrowly focused. This means that the sending and receiving antennas need to be aligned.

The unidirectional property has an obvious advantage. A pair of antennas can be aligned

without interfering with another pair of aligned antennas. The following describes some

characteristics of microwave propagation:

Microwave propagation is line-of-sight. Since the towers with the mounted antennas

need to be in direct sight of each other, towers that are far apart need to be very tall.

The curvature of the earth as well as other blocking obstacles do not allow two short

towers to communicate by using microwaves. Repeaters are often needed for long

distance communication.

Very high-frequency microwaves cannot penetrate walls. This characteristic can be

a disadvantage if receivers are inside buildings.

The microwave band is relatively wide, almost 299 GHz. Therefore wider subbands

can be assigned, and a high data rate is possible

Use of certain portions of the band requires permission from authorities.

Unidirectional Antenna:

Microwaves need unidirectional antennas that send out signals in one direction. Two

types of antennas are used for microwave communications: the parabolic dish and the horn.

A parabolic dish antenna is based on the geometry of a parabola: Every line parallel to

the line of symmetry (line of sight) reflects off the curve at angles such that all the lines

intersect in a common point called the focus. The parabolic dish works as a funnel, catching a

wide range of waves and directing them to a common point. In this way, more of the signal is

recovered than would be possible with a single-point receiver.

Outgoing transmissions are broadcast through a horn aimed at the dish. The

microwaves hit the dish and are deflected outward in a reversal of the receipt path.

Applications:

Microwaves, due to their unidirectional properties, are very useful when unicast (one-

to-one) communication is needed between the sender and the receiver.

Infrared: Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm

to 770 nm), can be used for short-range communication. Infrared waves, having high

frequencies, cannot penetrate walls. This advantageous characteristic prevents interference

between one system and another; a short-range communication system in one room cannot

be affected by another system in the next room. When we use our infrared remote control,

we do not interfere with the use of the remote by our neighbours. However, this same

characteristic makes infrared signals useless for long-range communication. In addition, we

cannot use infrared waves outside a building because the sun's rays contain infrared waves

that can interfere with the communication.

Applications:

Infrared signals can be used for short-range communication in a closed area using line-

of-sight propagation.

The infrared band, almost 400 THz, has an excellent potential for data transmission.

Such a wide bandwidth can be used to transmit digital data with a very high data rate. The

Infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has

established standards for using these signals for communication between devices such as

keyboards, mice, PCs, and printers. For example, some manufacturers provide a special port

called the IrDA port that allows a wireless keyboard to communicate with a PC. The standard

originally defined a data rate of 75 kbps for a distance up to 8 m. The recent standard defines

a data rate of 4 Mbps.